The present invention relates to a method for the preparation of fine globular particles of yttrium oxide or, more particularly, to a method for the preparation of fine particles of yttrium oxide having a globular or spherical particle configuration and having usefulness as a base material of a sintered body of yttrium oxide, sintering aid of various kinds of ceramic materials, base material of luminescent phosphors, coating agent on the particles of a luminescent phosphor and so on by virtue of absence of agglomeration and good dispersibility of particles.
While yttrium oxide powders are used widely in the above mentioned applications, it is desirable that the particles of the yttrium oxide powder therefor are as fine as possible to have a particle diameter in a submicron range and also the particles have dispersibility as high as possible. Such fine particles of yttrium oxide can be prepared in two different ways in the prior art. The first of the known methods therefor is to mechanically comminute separately prepared particles of yttrium oxide having a coarse particle diameter exceeding the desired submicron order by the use of a suitable pulverizing machine such as beads mills, optionally, with addition of a dispersing aid. This mechanical method, however, is industrially not feasible due to several problems and disadvantages. For example, it is by no means possible to avoid contamination of the yttrium oxide particles by the materials of the pulverizing machine and the beads as the pulverization medium as well as by the dispersing aid, when used. In addition, the efficiency of conventional pulverizing machines is generally not high enough when the desired particle size of the powder concerned is so fine as in the submicron range so that the productivity of the method is low as an industrial process.
As a natural consequence of the particle size reduction by a mechanical means, moreover, the particles obtained by this mechanical pulverization method usually have an irregular particle configuration with rugged surfaces as is evidenced by the discrepancy between the specific surface area obtained by the so-called BET method utilizing adsorption of nitrogen molecules and the imaginary specific surface area estimated by calculation from the value of the average particle diameter D.sub.50 of the particles assuming a perfectly spherical particle configuration and monodispersed distribution of the particle diameters, the former value usually being much greater than the latter. This discrepancy is presumably due to the fact that the specific surface area obtained by the BET method involves the additional areas due to the surface ruggedness of the particles as well as the internal surface areas provided by the cracks, fissures and pores occurring inside of the particles. In this regard, one of the evaluation items of a yttrium oxide powder is the discrepancy between or ratio of the specific surface area obtained by the BET method and the imaginary specific surface area calculated from the average particle diameter of the particles as mentioned above.
The second of the known methods for obtaining submicron particles of yttrium oxide, referred to as the urea method hereinafter, is a chemical method in which an aqueous solution of a water-soluble inorganic salt of yttrium is admixed with urea and the thus prepared reaction mixture is heated so that the urea in the solution is hydrolyzed into ammonium and carbonate ions which cause precipitation of fine particles of yttrium carbonate hydroxide having a submicron particle diameter followed by recovery of the precipitates and calcination thereof in an oxidizing atmosphere to convert the same into particles of yttrium oxide. A problem in this urea method is that, when yttrium oxide particles having an average particle diameter not exceeding 0.2 .mu.m are desired as the product, the concentration of the yttrium salt in the aqueous reaction mixture must be kept so low as not to exceed 0.01 mole/liter relative to yttrium. Accordingly, the urea method is undertaken only in a laboratory scale and is practically not industrial due to the low productivity.